The components of internal combustion engine fuel systems, particularly those engines that demand the supply of a regulated quantity of high pressure fuel to the cylinders, must ideally function without failure for thousands of hours. Plungers used to pressurize fuel and regulate the timing and/or the quantity of fuel delivered to the combustion chamber are integral fuel system components.
Fuel system plungers and other components are required to operate under extremely adverse environmental conditions over a wide range of operating temperatures in injectors, pumps and other assemblies where heavy mechanical loads may be applied in both axial and lateral or side directions to the plunger and/or other components. As the plunger reciprocates axially in its bore, the temperature of both the plunger and the bore wall increase. In a high pressure fuel system, the plunger may be required to reciprocate within a bore distorted by the axial and lateral or side loads applied to the plunger during engine operation. As a result, the original diametral clearance is not maintained, and the plunger is forced against the distorted bore wall during engine operation, which produces wear, scuffing, sticking, and, ultimately, failure. Additionally, low quality and contaminated fuels contribute to the creation of an adverse fuel system plunger operating environment. The types of fuels increasingly used in diesel engines, particularly fuels with low lubricity, alternative fuels and fuels which may be contaminated with water, usually require special parts or adjustment to enable the fuel system to function optimally. Ideally, reciprocating plunger components must be scuff-resistant to maintain efficient engine operation. The presence of fuel contaminants of all kinds, especially water, increases the propensity of conventional plungers to stick, scuff and seize.
The material used to form fuel system plungers has been modified throughout the years in an effort to make a plunger that is both scuff-resistant and wear-resistant and capable of functioning as required under the adverse conditions of the fuel system environment. Metal plungers have experienced unacceptable repair and failure levels. Moreover, the presence of third body debris interferes with efficient fuel system function when metal plungers are employed in high pressure fuel systems. Third body debris includes particles from the plunger or the fuel system component bore wall as well as fuel contaminants that are not intended to be present within the fuel system. These particles become embedded into the plunger surface, effectively decreasing the diametral clearance, and ultimately cause the plunger to be wedged to the bore wall so that the plunger cannot reciprocate in the bore and, thus, becomes friction welded or seizes. The reduction of fuel lubricity, which could be caused by water contamination of the fuel, and may be a characteristic of some alternative fuels, can also be a major factor contributing to the friction welding or seizure of a plunger within a fuel system component bore. Fuel system operation is, of course, prevented if this occurs.
In addition to permitting the fuel system plunger to reciprocate freely within its bore, the maintenance of an optimum, minimal diametral clearance between the plunger and the bore wall prevents excess fuel leakage. Some fuel leakage around the plunger is necessary for lubrication as the plunger reciprocates during operation of the high pressure fuel system. Excess fuel leakage, however, interferes with efficient fuel distribution and/or injection, which prevents efficient engine operation and may even completely prevent engine operation.
The prior art has proposed the use of wear-resistant materials and corrosion-resistant materials to form various structures and components of internal combustion engines. For example, U.S. Pat. No. 4,794,894 to Gill and U.S. Pat. No. 4,848,286 to Bentz are directed to ceramic tipped pivot rods, and U.S. Pat. No. 4,806,040 to Gill et al. is directed to a ceramic ball and socket joint. U.S. Pat. No. 4,266,729 to Kulke et al. discloses forming an injector valve needle tip and/or disc from a corrosion-resistant material, such as high quality steel, ceramic, or industrial glass. U.S. Pat. No. 5,409,165 to Carroll, III et al. discloses a wear-resistant fuel injector plunger assembly with a tip made of an impact wear-resistant ceramic and a body that may be made of a ceramic or a metal. The tip and body are specifically configured to be secured together to withstand the high temperatures and frictional forces produced by prolonged motoring. However, neither of these two patents nor any of the prior art of which the inventors are aware, addresses the specific problems of high pressure fuel system plunger scuffing and sticking which are encountered with available fuel systems, particularly those used in diesel engines. The prior art, moreover, also does not provide a high pressure fuel system plunger capable of maintaining an optimum minimal diametral clearance between the plunger and bore wall to prevent excessive fuel leakage.
The prior art, therefore, has failed to provide a plunger for a high pressure fuel system that is sufficiently scuff-resistant and wear-resistant, particularly when exposed to third body debris and such adverse operating conditions as low lubricity and contaminated fuels and heavy mechanical loads, to operate efficiently without requiring frequent repair or replacement. A need exists for such a high pressure fuel system plunger.